1. Field of Art
This invention relates to controlling magnetic recording head substrate bias voltage and more particularly relates to calculating a midpoint voltage of a transducing element in the head and calculating a preferred substrate bias voltage.
2. Background Technology
Since the IBM 726 in 1952, the magnetic recording industry has continuously improved upon the performance and capacity of drives and media to accommodate the insatiable demand for larger and superior storage.
Tape drives and hard disk drives employ heads having write and read transducing elements to record and read data on their respective magnetic physical media, but in different ways. A tape drive head usually consists of multiple write and read transducers laid out perpendicular to the tape media to access multiple tracks at one time, whereas a hard disk drive typically employs heads having single read and write transducers per disk surface.
Very early on a single transducer was used for both writing and reading. These drives use heads having inductive transducers, which are essentially spiral coils wrapped between two layers of magnetic material. Writing to the physical media is achieved by applying an electrical current through the coil to produce a magnetic field, which forms a series of magnetic flux patterns on the surface of the physical media. The direction of the magnetic field depends on the direction of the applied current. Reading from the physical media involves the opposite principle: applying a magnetic field to the coil. In other words, gliding the head over the recorded magnetic flux pattern on the physical media causes an electrical current to flow in the coil. The electrical current corresponds to the orientation of the previously recorded magnetic field, where, in binary terms, a transition indicates a 1, and no transition implies a 0.
As recording technology advances, using a single coil for both writing and reading limits performance, since in many cases improving the inductive coil for reading adversely affects the writing performance, and vice-versa. Furthermore, in order to increase the track density, data is written to a wider data track and read from a narrower region of the written track, thereby minimizing misregistration between readers and written tracks. Separating the read-write transducer into separate read and write transducers allowed each to be optimized solely for their specific function.
In order to satisfy demand for increased areal density, modern drives switched from inductive read elements to magneto-resistive (MR) read elements and more recently to giant magneto-resistive (GMR) elements. Generally, “head” refers to the entire structure consisting of substrate, closure, transducers, etc. However, it is common practice also to refer to the MR sensors and write transducers as “heads,” and this practice will be followed hereinafter. The correct technical name for first-generation MR heads is anisotropic magneto-resistive (AMR), but traditionally they have just been called “magneto-resistive” (MR). Unlike the induced currents of an inductive head, MR heads work via the MR effect, where MR material changes electrical resistance in the presence of a magnetic field and, thus, detects transitions in the magnetic field representative of recorded data. MR heads contain a sensing layer, or stripe, which includes the MR material. A bias current is applied to the stripe and changes in the voltage across the sensor are measured. The total DC voltage across the stripe is the product of the bias current times the stripe resistance. The stripe resistance varies with the stripe magnetization, which is a combination of internal magnetization and external magnetization. As is well known, the voltage drop across MR heads is typically in the range of several tenths of volt or higher. This dc voltage appears at the surface of the head whenever the bias is turned on.
For proper read back detection, the MR sensors require magnetically permeable shields. The shields are fabricated on both sides of the sensor stripe. The two shields form the magnetic sense gap. The spacing between the shields sets the frequency response of the head. The sensor is located approximately in the mid-plane between the shields. As is well known in the art, the shield spacing must be in the range of 0.1 to 1.0 micrometer. The insulation layer between shields and sensor is even smaller, often only a few hundred Angstroms or less. As is well known in the art, the shields may be electrically connected to the MR leads via thin film resistors. Further, writer poles may be electrically connected to a neighboring MR shield pair. When so connected, all poles and shields are clamped to a voltage that is derived from the MR sensor.
Modern linear tape drives typically write multiple tracks simultaneously on each pass of the bidirectional tape media. Additionally, the recording heads in tape drives usually have two modules, each of which contains both read and write elements. The modules face each other so that an MR head read element on one head module faces a Thin Film Inductive (TFI) head write element on the opposite head module. This way, the data that is being written with a TFI write element from one module can be verified by the MR read element on the opposite module on each pass of the tape.
In addition to specialized read and write elements, many tape drives employ servo control, or “servoing”, via a servo read element in order to keep the read and write elements in line with the tape media. The servo read elements are similar to data readers. Advanced tape drives may employ a timing-based servo (TBS) to provide very precise position information to the drive. Tapes are factory-formatted with the TBS pattern, in which the obliquely written patterns are used to indicate position information. For instance, the physical media may be divided into four separate bands. Each band may be further divided. For example, each band may have twelve TBS positions, or six for each direction of tape travel, which would give a total of 24 unique positions for writing data to the tape.
Tape drives may incorporate redundant servo elements so that if a servo reader becomes temporarily defective or a portion of the pre-written servo track is corrupted, the redundant servo reader will keep the head in line with the track locations on the tape for the duration of the defect.
All the heads of a head module, the read, write and servo heads, are fabricated on a head substrate. The choice of the substrate has varied over the years, but in all cases, in order to protect the small, sensitive magnetic recording elements, the substrates are made of hard materials which have minimum wear when rubbed by tape. One example of a material is a hard ferrite which may also serve as a magnetic shield for the sensors. Another choice, and the one used in many hard disk drive and tape drive products, is a hard ceramic made of HIPed Alumina and Titanium Carbide (AlTiC). In the case of ferrites, the substrate is insulative, while in the case of AlTiC, the substrate may be conductive. In this case, as is well known, it is preferable to clamp the substrate voltage to a pre-set value.
The response of an MR read element to a magnetic field, defined as the device transfer curve, is non-linear. Therefore, in order to acquire an undistorted reproduction of a recorded magnetic field, the magnetization of the sensing layer of the MR read element is biased. As is well known in the art, the bias current through the MR read element is programmed so as to minimize the signal distortion. However, this may result in some MR elements getting programmed with an excessively high bias current. Raising bias current increases both MR temperature and voltage and shield and write pole voltages.
The bias current is programmed based on performance parameters of each MR head, typically in a range of 4-15 milliamps. The MR head substrate bias voltage is set to a preset voltage value, typically 0, 1.5, 3.0 volts, achieved with a fixed voltage divider connected to a supply voltage. It is determined pre-assembly and, once the drive is assembled, is not adjusted despite natural degradation in the MR head elements. Since modern linear tape drives have 8, 16, or more active channels, there may be as many different bias current values in each module.
Furthermore, the reader elements are resistors and conduct electric current. They operate at a voltage that may be very different than the triboelectric voltage on the surface of the magnetic physical media. This difference is associated with such phenomena as electro-chemical depositions upon the magnetic physical media and stripe oxidation, as well as other deleterious effects. Besides causing resistance increases in the sensing layer and consequential degradation of the sensing amplitude, such variations have been the cause of “head shorting,” where the physical media shorts the MR sensor to its shields. In addition, parasitic conductive paths between head substrate and MR shields may develop over time with the passage of tape over the head. If the resistance of these parasitic paths is low enough, the substrate voltage itself can change, where the magnitude of change depends on the substrate bias circuit and other resistors. Furthermore, under normal operating conditions, current tape drives are unable to at detect and adapt to these measurable changes in the performance of head elements.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that overcome the limitations of conventional substrate biasing method. In particular, such an apparatus, system, and method would beneficially control substrate bias dynamically, thereby avoiding the drawbacks associated with a fixed substrate bias voltage. The apparatus, system, and method would also beneficially provide in-drive recalibration in the event of a variation in resistance in any of a plurality of head elements associated with a module.